1. Field of the Invention
The present invention relates to a surface acoustic wave device used, for example, for a resonator and a band pass filter, and more particularly, relates to a surface acoustic wave device having a structure in which an insulating layer is arranged so as to cover electrodes.
2. Description of the Related Art
A DPX and an RF filter used for mobile communication systems have been required to satisfy broadband properties and superior temperature properties. Heretofore, in a surface acoustic wave device used for a DPX and an RF filter, a piezoelectric substrate of 36° to 50° rotated Y-plate X-propagation LiTaO3 has been used. This piezoelectric substrate has a frequency-temperature coefficient of approximately −40 ppm/° C. to −30 ppm/° C. Hence, in order to improve the temperature properties, a method has been used in which a SiO2 film having a positive frequency-temperature coefficient is formed on the piezoelectric substrate so as to cover an IDT electrode. In FIGS. 18A-18D, one example of a manufacturing method of this type of surface acoustic wave device is shown.
As shown in FIG. 18A, a resist pattern 52 is formed on a piezoelectric substrate 51 except for portions at which the IDT electrode is to be formed. Next, as shown in FIG. 18B, in order to form the IDT electrode, an electrode film 53 is formed over the entire surface. Subsequently, by using a resist stripping solution, the resist patter 52 and the metal film provided thereon are removed. As described above, as shown in FIG. 18C, an IDT electrode 53A is formed. Next, as shown in FIG. 18D, a SiO2 film 54 is formed so as to cover the IDT electrode 53A.
On the other hand, for a purpose different from that for improving frequency-temperature properties described above, a manufacturing method of a surface acoustic wave device is disclosed in Japanese Unexamined Patent Application Publication No. 11-186866 (Patent Document 1) in which an insulating or an semi-conductive protective film is formed so as to cover an IDT electrode of the surface acoustic wave device. FIG. 19 is a schematic cross-sectional view of the surface wave device described in this prior art. In a surface acoustic wave device 61, an IDT electrode 63 of Al or an alloy primarily made of Al is formed on a piezoelectric substrate 62. In a region other than that at which the IDT electrode 63 is provided, an insulating or an anti-conductive inter-electrode-finger film 64 is formed. In addition, so as to cover the IDT electrode 63 and the inter-electrode-finger film 64, an insulating or an anti-conductive film 65 is formed. In the surface acoustic wave device 61 described in this prior art, the inter-electrode-finger film 64 and the protective film 65 are formed of an insulating material, such as SiO2, or an anti-conductive material, such as silicon. In this method, with the formation of an inter-electrode-finger film 64, degradation in properties caused by discharge between the electrode fingers induced by the pyroelectric properties of a piezoelectric substrate 62 is suppressed.
In addition, in Japanese Unexamined Patent Application Publication No. 61-136312 (Patent Document 2), a one-port type surface acoustic wave resonator is disclosed in which after electrodes made of a metal such as aluminum or gold are formed on a piezoelectric substrate made of quartz or lithium niobate, and a SiO2 film is further formed, planarization of the SiO2 film is performed. In this case, due to the planarization, superior resonant properties are obtained.
As shown in FIGS. 18A-18D, in a conventional manufacturing method of a surface acoustic wave device in which a SiO2 film is formed in order to improve the frequency-temperature properties, the surface height of the SiO2 film 54 disposed at locations at which the IDT electrode is present and the surface height at which the IDT electrode is not present are different from each other. Thus, the presence of irregularities of the surface of the SiO2 film 54 causes a problem in that the insertion loss is degraded. In addition, as the thickness of the IDT electrode is increased, the irregularities are increased. Accordingly, the thickness of the IDT electrode cannot be sufficiently increased.
On the other hand, in the surface acoustic wave device described in Patent Document 1, after the inter-electrode-finger film 64 is formed between electrode fingers of the IDT 63, the protective film 65 is formed. Accordingly, the surface of the protective film 65 can be planarized.
However, according to the structure described in Patent Document 1, the IDT electrode 63 is formed of Al or an alloy primarily composed of Al. Although the inter-electrode-finger film 64 is formed so as to be in contact with this IDT electrode 63, a sufficient reflection coefficient cannot be obtained at the IDT electrode 63. As a result, for example, there has been a problem in that ripples are generated in the resonant properties.
Furthermore, in the manufacturing method described in Patent Document 1, prior to the formation of the protective film 65, the resist formed on the inter-electrode-finger film 64 must be removed using a resist stripping solution. However, in this step, the IDT electrode 63 may be corroded by a resist stripping solution. Thus, as a metal forming the IDT electrode, a metal that is susceptible to corrosion cannot be used. That is, the type of metal forming the IDT electrode is limited.
On the other hand, in the one-port type surface acoustic wave resonator described in Patent Document 2, it has been disclosed that quartz or lithium niobate is used as the piezoelectric substrate and the electrode is formed of aluminum or gold. However, the situation in which an electrode made of Al is formed on a quartz substrate is only described as an example. That is, a surface acoustic wave device using another substrate material and/or another metal material has not been specifically described.